In general, an amplifier for communication equipment is required to have high efficiency and it is known that a matching circuit is used to achieve high efficiencies of an amplifier as disclosed in a Japanese Published Unexamined Patent Application of No. 32335/95. When signals are amplified by an amplifier, such undesired signal components as noise and the like that exist outside the frequencies intended for Amplification are also amplified at the same time. Because of this, a filter is connected to the output side of an amplifier to attenuate undesired signal components such as noise and the like.
One of the problems involved with the foregoing system has been that attenuation of the noise amplified by an amplifier and additional noises has to be achieved by a filter alone situated behind the amplifier in the system, thereby requiring the filter to have large attenuation by using many resonators resulting in a bulky filter. In addition, the loss in a pass-band of the filter connected behind the amplifier tends to increase, thus requiring the amplifier to output more power with resultant hindrance to the efficiency improvement of the amplifier.
More specifically, the relations involved with band-pass filter attenuation vs. frequency characteristics, frequency spacings in transmitting signals and receiving signals, and the like will be explained with reference to FIG. 14. FIG. 14 shows a case wherein a receiving signal band is located in a higher frequency band than a transmitting signal band. In general, with portable telephones, a plurality of both transmitting signal frequencies and receiving signal frequencies are prepared within a given frequency band, thereby communications being performed between transmitting signals and receiving signals through a step of selecting a pair of frequencies, the frequency spacing of which maintains a constant value. Also, signals that have passed a filter behind an amplifier are needed to suppress undesired signal components such as noise and the like existing in the receiving signal frequencies in order to prevent a reduction in receiving sensitivity caused by an infiltration of undesired signals into a receiving section. The general characteristics required of a filter that satisfies the foregoing requirement are to have a small amount of attenuation in the transmitting frequency band and a large amount of attenuation in the receiving frequency band, and to make the attenuation in the transmitting frequency band stay lower than the tolerated attenuation level of the transmitting signal at the highest frequency and the attenuation in the receiving frequency band maintain the needed attenuation of the receiving signal at the lowest frequency. As a result, the filter characteristics are required to have a large amount of attenuation change for frequency changes, thereby causing, in general, the filter characteristics to show large attenuation in transmitting signals and reduced amplification efficiencies.